Sub test()
testString = "firstpart3secondpart"
For i = 1 To Len(testString)
    If IsNumeric(Mid(testString, i, 1)) Then
        part1 = Left(testString, i - 1)
        part2 = (Right(testString, Len(testString) - i))
    End If
Next i
MsgBox (part1)
MsgBox (part2)
End Sub